. Principles of modern biology. Biology. Fig. 24-5. Fatigue. Many successive twitches of an excised muscle, stimulated about 30 times per minute, are more or less superimposed. The earlier contrac- tions reach higher levels, and the relaxations occur more quickly. Finally the capacity to contract is abol- ished completely. (From Gerard, The Body Functions. Permission of John Wiley and Sons, Inc.) series of action potentials can be detected in the muscle, so long as the tetanus continues. Gradation of Contractions. If a single fiber (Fig. 24-6) is isolated from the many that compose even the sm
. Principles of modern biology. Biology. Fig. 24-5. Fatigue. Many successive twitches of an excised muscle, stimulated about 30 times per minute, are more or less superimposed. The earlier contrac- tions reach higher levels, and the relaxations occur more quickly. Finally the capacity to contract is abol- ished completely. (From Gerard, The Body Functions. Permission of John Wiley and Sons, Inc.) series of action potentials can be detected in the muscle, so long as the tetanus continues. Gradation of Contractions. If a single fiber (Fig. 24-6) is isolated from the many that compose even the smallest whole muscle, the fiber contracts with maximal force each time it is excited successfully, regardless of whether the exciting stimulus is relatively strong or weak. This all-or-none law holds true for the single fiber, but it does not apply to the muscle as a whole. The intact muscle gives graded contractions according to the strength of the stimulus applied. Weaker stimuli do not arouse all the fibers of the muscle; and a maximal contraction, in the case of either a tetanus or a twitch, is ob- tained only when the stimulus is strong enough to bring all the fibers of the whole muscle into play. Chemical Aspects of the Contractile Process. Even when completely deprived of oxygen, a muscle can perform many twitches and can sustain a fairly long tetanus. But without oxygen, fatigue sets in prematurely; or to state the matter more precisely, under aerobic conditions a muscle can do about four times more work than under anaerobic conditions. Chemical analysis of a totally fatigued mus- cle reveals a number of changes in the tissue. The glycogen of the tissue, which in a rested muscle amounts to some 3 percent by weight, has declined considerably, and the quantity Myofibrils (t-2.« ^ESKjJ^Mycle't of niuAclt fvi.^i* 1 S .:sfrf!lf£&— _Musc!e fiber crushed Showing -SsrcoUmma ( IaO sC-rmaetive tissue « lb , perl-myali Blum end of fiber. Fig. 24-6. Group of fibers of skele
Size: 1088px × 2297px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
